Size-Dependent Mechanical Properties of a Metal-Organic Framework: Increase in Flexibility of ZIF-8 by Crystal Downsizing.

Size engineering is an emerging strategy to modulate the mechanical properties of crystalline materials. Herein, micro- and nanodimensional single crystals of the prototypical metal-organic framework (MOF) ZIF-8 are generated using solvothermal and solution methods, respectively. Atomic force microscopy-based nanoindentation technique was used to measure the Young's modulus values of micro- and nanodimensional individual ZIF-8 crystals. We demonstrate that crystal downsizing to nanoscale dimensions results in a 40% reduction in crystal stiffness. The change is attributed to a greater contribution of surface effects to the physical properties of nanocrystalline ZIF-8. The observed change in the mechanical properties may be used to explain reported size-dependent changes in gas adsorption of ZIF-8, thought to be a result of differences in framework flexibility at the nanoscale. Our work provides an important example on how downsizing of crystalline metal-organic materials can give rise to specific and tunable physical properties.

Effects of a Two-Step Silver Diamine Fluoride Varnish on Shear Bond Strength of Restorations, Dentin and Enamel Hardness, and Biofilm Formation.

INTRODUCTION: Dental caries are a limiting factor in maintaining dental and medical readiness in the military. Untreated dental caries can lead to dire health consequences. Consistent and comprehensive access to dental care is often limited due to the intensive operational demands on our nation's warfighters. The standard of care for dental caries is a surgical model where diseased tooth tissue is surgically removed and restored with appropriate restorative materials. While effective, it is not practical in the military operational environment, especially under time constraints. Dental restoratives offer military personnel a simple and preventive treatment of dental caries and are suitable as self-applied first aids. The purpose of this study was to measure the shear bond strengths of two dental restorative materials to human teeth paired with two different fluoride treatments and the hardness and biofilm formation on teeth after applying the fluoride varnishes. MATERIALS AND METHODS: Specimens were made of human molar teeth treated with each of the following four materials: glass ionomer cement GC Fuji II LC Capsules, Filtek Z250, Riva Star steps 1 and 2, or Mark3 NaF varnish. Step 1 of Riva Star consists of silver diamine fluoride and step 2 contains potassium iodide. On human molar slabs, 10 circular specimens of 5 cm in diameter were prepared with restoratives according to manufacturer procedures. Etch-Rite and a proprietary aluminum chloride-based cavity conditioner were used as etchants on tooth surfaces for the Filtek Z250 and glass ionomer cement, respectively. After at least 24 hours underwater, each assembly was removed, and the shear bond strength of the adhesive was measured according to International Organization for Standardization (ISO) 29022.The hardness was measured according to ISO 14233. Hardness measurements were performed before varnish application, then after storage in an incubator at 37 °C for 4 hours in a demineralization solution (pH = 4.5), and after 1 day in a mineralization solution (pH = 7). A crystal violet staining assay was used to measure biofilm formation of Streptococcus mutans bacteria on human molar teeth after the application of fluoride varnish. RESULTS: We report a 16% increase in shear bond strength of the Filtek Z250/Riva Star coupled treatment compared to the Filtek Z250/Mark3 NaF coupled treatment. We also demonstrate a significant 84% decrease in bond strength with a GC Fuji II LC/Mark3 NaF treatment compared to control (P = .0002), while Riva Star remains statistically unchanged. Enamel and dentinal hardness are significantly improved when Riva Star is applied compared to NaF varnish. A 25%-35% (P < .0001) decrease in oral biofilm formation was observed on samples where a Riva Star or NaF varnish was applied. CONCLUSIONS: Mechanical and antimicrobial testing indicated Riva Star, compared favorably with and in some cases, performed better in the laboratory than a Mark3 NaF varnish. Hardness measurements indicated Riva Star is more effective in dentin tubule occlusion compared to NaF varnish. Our findings help provide practical suggestions to dental treatment, particularly to the unique dental environments seen in the military. Riva Star may be used as an adjunctive treatment prior to placing a final restoration. This study supports the use of Riva Star in conjunction with GC Fuji II LC or Filtek Z250 restorative materials, making it a promising treatment in military dental applications.

Correction: Mechanical rigidity of a shape-memory metal-organic framework increases by crystal downsizing.

Correction for 'Mechanical rigidity of a shape-memory metal-organic framework increases by crystal downsizing' by Al A. Tiba et al., Chem. Commun., 2021, 57, 89-92, DOI: 10.1039/D0CC05684G.

Mechanical rigidity of a shape-memory metal-organic framework increases by crystal downsizing.

Soft porous nanocrystals with a pronounced shape-memory effect exhibit two- to three-fold increase in elastic modulus compared to the microcrystalline counterpart as determined by atomic force microscopy nanoindentation. The increase in rigidity is consistent with the known shape-memory effect displayed by the framework solid at the nanoscale. Crystal downsizing can offer new avenues for tailoring the mechanical properties of metal-organic frameworks.